The present invention relates to a split-gate flash memory cell and manufacturing method thereof; and, more particularly, to a split-gate flash memory cell with a peak floating gate and manufacturing method thereof.
The shape and size of different portions of memory cells affects the performance of the memory cells differently. Thus, with the one-transistor memory cell, which contains one transistor and one capacitor, many variations of this simple cell have been advanced for the purposes for shrinking the size of the cell and, at the same time, improve its performance. The variations include different methods of forming capacitors, with single, double or triple layers of polysilicon, and different materials for the word and bit lines.
Memory devices include electrically erasable and electrically programmable read-only memories (EEPROMs) of flash electrically erasable and electrically programmable read-only memories (flash EEPROMs). Many types of memory cells for EEPROMs or flash EEPROMs may have source and drain regions that are aligned to a floating gate or aligned to spacers.
Referring to FIG. 1, there is shown a conventional split-gate flash memory cell disclosed in U.S. Pat. No. 5,970,371.
On a substrate 10 with a plurality of active regions, e.g., a source 11 and a drain 13, already defined, a tunnel oxide layer 20 is formed. A polysilicon layer 30 is next deposited over the tunnel oxide layer 20. A portion of the polysilicon layer 30 is oxidized by employing a local oxidation of silicon (LOCOS) process to form a LOCOS polyoxide 35. The LOCOS polyoxide 35 is used as a hard mask to etch the remaining portion of the polysilicon layer 30 not covered by the LOCOS polyoxide 35. Since the shape of the LOCOS polyoxide 35 is generally rounded even after performing over-etch, the polysilicon layer 30 is modified as a floating gate 30 with a sharp beak 37. After an inter-gate oxide layer 60 is deposited, a control gate 70 of polisilicon is formed over a portion of the inter-gate oxide layer 60.
In order to program the split-gate flash memory cell, charges in the source 11 are transferred through the tunnel oxide layer 20 to the floating gate 30. On the other hand, in order to erase the split-gate flash memory cell, charges in the floating gate 30 are removed through the inter-gate oxide layer 60 to the control gate 70.
Since, however, the LOCOS polioxide 35 is rounded to form the sharp beak 37 of the floating gate 30, the thickness of oxide between the floating gate 30 and the control gate 70 is not uniform. Further, since the control gate 70 and the floating gate 30 are formed separately, the control gate 70 dose not cover the floating gate 30 completely to reduce a coupling ratio between the floating gate 30 and the control gate 70 and, therefore, the programming/erasing efficiency of the split-gate flash memory cell is considerably reduced.
It is, therefore, a primary object of the present invention to provide a split-gate flash memory cell with a peak floating gate and manufacturing method thereof to improve a coupling ratio between the peak floating gate and the control gate.
In accordance with one aspect of the present invention, there is provided a method for manufacturing a split-gate flash memory cell, the method comprising the steps of:
(a) providing a substrate;
(b) forming a tunnel oxide layer over the substrate;
(c) forming a peak floating gate layer of conducting material over a portion of the tunnel oxide layer, wherein the peak floating gate layer has a peak structure thereon;
(d) coating an inter-gate insulating layer over the peak floating gate layer and the remaining portion of the tunnel oxide layer and spreading a control gate layer of conducting material over the inter-gate insulating layer;
(e) defining a control gate pattern over the control gate layer;
(f) etching down the control gate layer, the inter-gate insulating layer, the peak floating gate layer and the tunnel oxide layer sequentially to the substrate by using the control gate pattern to generate a control gate, a inter-gate insulating region, a peak floating gate and a tunnel oxide region; and
(g) defining a source and a drain adjoining the tunnel oxide region by using a self-align technique.
In accordance with another aspect of the present invention, there is provided a split-gate flash memory cell comprising:
a substrate defined with a source, a drain and a channel region between the source and the drain;
a tunnel oxide region formed over the channel region;
a peak floating gate of conducting material over a portion of the tunnel oxide region, wherein the peak floating gate has a peak structure thereon;
an inter-gate insulating region formed over the peak floating gate and the remaining portion of the tunnel oxide region; and
a control gate of conducting material formed over the inter-gate insulating region,
wherein the control gate, the inter-gate insulating region, the peak floating gate and the tunnel oxide region are defined with a mask which has a same size as the channel region.